This invention relates to laminated materials possessing a high gas-impermeability and an advantageous workability. More particularly, this invention relates to laminated materials which are prepared by laminating a layer of vinylidene chloride type resin containing a low molecular weight plasticizer with a layer containing a functional oligomer, fat and oil, etc. with or without having a thin adhesive layer interposed therebetween. These laminates are characterized by the improved gas-impermeability imparted to the laminated materials owing to transfer of the low molecular plasticizer from the vinylidene chloride resin layer into the resin layer containing the functional oligomer, oil and fat, etc.
Heretofore, compositions prepared by mixing vinylidene chloride type resins, namely vinylidene chloride-based copolymers, with suitable amounts of modifiers such as plasticizers and stabilizers were either molded alone or laminated with other resins to produce films, sheets and containers, which have found extensive utility as food packaging materials, excellent for intercepting oxygen and water vapour.
Foodstuffs which are highly perishable must be protected from contact with oxygen. Dry foodstuffs must be protected from even a slight absorption of moisture and retorted foodstuffs. However, there are cases where the ability of food packaging materials to intercept oxygen and water vapour does not prove sufficient. Thus, a need has been felt for development of new packaging materials capable of offering enhanced protection against such gaseous substances. One possible way of providing a sufficiently high gas-barrier property for these packaging materials is to increase the vinylidene chloride content of the copolymer composition. In these cases, however, the packaging materials become susceptible to thermal decomposition and discoloration because of the inevitable approximation of the decomposition temperature and the melting point of the copolymer. In order that the conventional compositions may be advantageously molded on a commercial scale by the melt extrusion method using ordinary screw type extruders without entailing thermal decomposition or discoloration, therefore, it has been necessary for compositions to contain considerable amounts of modifiers such as a plasticizer and a stabilizer. These modifiers lower the melting point and improve the thermal stability but, on the other hand, deteriorate the gas-impermeability of such a composition directly in proportion to the amount used. It has, accordingly, been customary to incorporate the modifiers in amounts from 4 to 10% by weight to suit particular uses and purposes.
Heretofore, it has been generally held that the lower limit of the combined amount of these modifiers is to be fixed at 4% by weight for the purpose of permitting the commercial manufacture of molded products to be carried out with ample smoothness of operation, and at 3% by weight for the purpose of enabling the manufacture to be effected, though at the expense of the smoothness of operation. If the amount is below the lower limit, the manufacture becomes extremely difficult and practically infeasible.
The present invention, therefore, aims chiefly to eliminate such limits imposed on the gas-impermeability and at the same time satisfy the workability requirement. For the purpose of enabling a layer of vinylidene chloride type resin contained in laminated materials to retain the outstanding gas-impermeability of the vinylidene chloride type resin, various modifiers and liquid substances are not added at all or are added only in the minimum allowable amount into another layer adjoining this vinylidene chloride resin layer lest these modifiers and liquid substances should shift into the vinylidene chloride resin layer. When a layer of the vinylidene chloride type resin is laminated with another layer of ordinary flexible vinyl chloride type resin containing as much as ten percent of a low molecular plasticizer, for example, a large portion of the plasticizer present in the flexible vinyl chloride resin layer shifts into the vinylidene chloride type resin and consequently deteriorates its gas-impermeability. Thus, it has been found necessary to apply the laminated material to limited uses which require no appreciable preserving property or that the layer of vinylidene chloride type resin in the laminated material should possess a considerably greater thickness.